Photoinduced modifications of NAD(P)H attributed autofluorescence of CHO cells in a single-beam gradient force optical trap (optical tweezers) were studied. No strong effect of the 1064 nm trapping beam on fluorescence intensity and spectral characteristics was found, even for power densities up to 70 MW/cm2 . In contrast, 760 nm microirradiation resulted in a significant fluorescence increase, probably indicating cell damage due to absorption by heme-containing molecules. UVA exposure of the trapped cells generated within seconds an initial fluorescence decrease , followed by a significant increase up to 5X of the value prior to irradiation. The UVA-induced modifications reflect NAD(P)H autooxidation and irreversible cell damage due to oxidative stress.